This invention relates to the polymerization and copolymerization of a mono-1-olefin monomer, such as ethylene, with a higher alpha-olefin comonomer.
Supported chromium catalysts long have been a dominant factor in the production of high density olefin polymers, such as polyethylene. As originally commercialized, these catalyst systems were used in solution polymerization processes. However, it became evident early that a slurry process was a more economical route to many commercial grades of olefin polymers, that is, a polymerization process carried out at a temperature low enough that the resulting polymer is largely insoluble in the diluent.
It is well known that mono-1-olefins, such as ethylene, can be polymerized with catalyst systems employing vanadium, chromium or other metals on a support, such as alumina, silica, aluminum phosphate, titania, zirconium, magnesium and other refractory metal supports. Initially, such catalyst systems primarily were used to form homopolymers of ethylene. Soon copolymers were developed wherein comonomers such as propylene, 1-butene, 1-hexene or other higher mono-1-olefins were copolymerized with ethylene to provide resins tailored to specific end uses.
Often, high density and/or high molecular weight copolymers can be used for blow molding applications because the blow molding process enables rapid processing into a desired molded product. Theoretically, any type of resin can be made to flow more easily by merely lowering the molecular weight, (i.e., by raising the melt index.) However, this is rarely practical because of other penalties that occur because of a higher melt index (MI). A higher melt index can result in a decrease in melt strength, which can cause a parison to tear or sag during extrusion because the parison is unable to resist its own weight. As used in this disclosure, a parison is an extruded cylinder of molten polymer before it is blown by air pressure to fill a mold. Additionally, a higher MI can cause bottle properties such as environmental stress crack resistance (ESCR) and impact strength to decrease. One of the most prevalent problems associated with raising the MI is an increase of the amount of swell exhibited by the resin as it exits the die.
Two kinds of swell are critical during blow molding. These are xe2x80x9cweight swellxe2x80x9d and xe2x80x9cdiameter swellxe2x80x9d; the later also is referred to herein as xe2x80x9cdie swellxe2x80x9d. As polymer, or resin, is extruded under pressure through a die opening and into a mold, a polymer has a tendency to swell as it exits the die. This is known as weight swell and is determinative of the thickness of bottle wall, as well as the overall weight of the resultant blow molded product. For example, a resin which is extruded through a 0.02 inch die gap might yield a bottle wall thickness of 0.06 inches, in which case the weight swell is said to be 300%. A resin that swells too much can produce a bottle with too thick of a wall. To compensate, the die opening or gap can be narrowed by manual adjustment. However, any decrease in die gap can increase the resistance to the flow of the resin through the die. Narrower die gaps can result in higher shear rates during extrusion which also can increase in melt fracture leading to a rough bottle surface. Thus, a resin which can be described as easily processable must exhibit low weight swell, which allows a wide die gap.
Diameter, or die, swell refers to how much the parison flares out as it is extruded from the die. For example, a resin extruded through a circular die of one (1) inch diameter can yield a parison tube of 1.5 inches in diameter; the die swell is said to be 50%. Die swell is significant because molds usually are designed for a certain amount of flare; too much die swell can interfere with molding of a bottle handle. A high degree of weight swell often causes high die swell because of the narrow gap that accompanies it. Unfortunately, increasing the melt index of a resin usually increases both weight swell and die swell of the polymer. Thus, as used herein, a resin which is considered easily processable also should exhibit low die swell.
Therefore, it is an object of this invention to provide an improved olefin polymerization process.
It is another object of this invention to provide a process to produce copolymers of ethylene and mono-1-olefins that can be processed at increased production rates and have a decreased weight swell.
It is still another object of this invention to provide a process to produce copolymers of ethylene and mono-1-olefins that can be processed at increased production rates and have a decreased die swell.
In accordance with this invention, herein is provided a polymerization process comprising contacting under slurry polymerization conditions at a temperature within a range of about 200xc2x0 F. to about 226xc2x0 F. (about 93xc2x0 C. to about 108xc2x0 C.) in an isobutane diluent:
a) ethylene monomer;
b) at least 1 mono-1-olefin comonomer having about three to eight carbon atoms per molecule;
c) a catalyst system comprising chromium supported on a silica-titania support, wherein said support comprises from about 1 to about 10 weight percent titanium, based on the weight on the support, wherein said catalyst system has a pore volume within a range of about 0.5 to about 1.3 ml/g, a surface area within a range about 150 to 400 m2/g, and said catalyst system has been activated at a temperature within a range of about 800xc2x0 F. to about 1300xc2x0 F. (about 427xc2x0 C. to about 704xc2x0 C.);
d) a trialkylboron compound; and
e) recovering an ethylene/mono-1-olefin copolymer.
In accordance with another embodiment of this invention, a copolymer comprising ethylene and a mono-1-olefin having from about 3 to about 8 carbon atoms carbon atoms per molecule is provided, wherein said copolymer has a high load melt index (HLMI) within a range of about 10 to about 80 g/10 minutes, a density within a range of about 0.95 to 0.96 g/cc, a weight swell lower than about 380%, and a die swell lower than about 43%. An environmental stress crack resistance (ESCR) of greater than about 200 hours, a Mw/Mn of greater than about 12 and the onset of melt fracture of greater than about 2000 secxe2x88x921.
As used in the description herein, the terms xe2x80x9ccogelxe2x80x9d and xe2x80x9ccogel hydrogelxe2x80x9d are arbitrarily used to describe cogelled silica and titania. The term xe2x80x9ctergelxe2x80x9d is used to describe the product resulting from gelation together of silica, titania, and chromia. References to xe2x80x9csilicaxe2x80x9d mean a silica-containing material generally comprised of 80 to 100 weight percent silica, the remainder, if any, being selected from alumina, boria, magnesia, thoria, zirconia, or mixtures thereof. Other ingredients which do not adversely affect the catalyst or which are present to produce some unrelated results also can be present.
The support for the catalyst of this invention must be a cogel of silica and a titanium compound. Such a cogel hydrogel can be produced by contacting an alkali metal silicate such as sodium silicate with such as an acid, carbon dioxide, or an acidic salt. The preferred procedure is to utilize sodium silicate and an acid such as sulfuric acid, hydrochloric acid, or acetic acid, with sulfuric acid being the most preferred due to less corrosivity and greater acid strength. The titanium component must be coprecipitated with silica and thus most conveniently the titanium compound will be dissolved in the acid or alkali metal silicate solution.
The titanium compound preferably is incorporated with the acid. The titanium compound can be incorporated in the acid in any form in which it will be subsequently incorporated in the silica gel formed on combination of the silicate and the acid (preferably by means of adding the silicate to the acid) and from which form it is subsequently convertible to titanium oxide on calcination. Suitable titanium compounds include, but are not limited to, halides such as TiCl3 and TiCl4, nitrates, sulfates, oxalates and alkyl titanates. In instances where carbon dioxide is used, the titanium, of course, must be incorporated into the alkali metal silicate itself. Also with acidic salts it is preferred to incorporate the titanium compound in the alkali metal silicate and in such instances, preferred titanium compounds are water soluble materials which do not precipitate the silicate, i.e. are those convertible to titanium oxide on calcination such as, for example, K2TiO(C2O4)2H2O (titanium potassium oxalate); (NH4)2TiO(C2O4)2H2O and Ti2(C2O4)3H2O.
The titanium compound preferably is present in an amount within the range of about 1 to about 10, preferably about 1 to about 8, and most preferably about 2 to about 8 weight percent, calculated as titanium, based on the weight of the cogel. The preferred titanium ranges result in a catalyst system that can have improved activity and a higher melt index polymer.
The catalyst of this invention must contain a chromium compound. The chromium compound can be incorporated in any of several separate ways. First, a tergel can be prepared wherein the chromium compound, as well as a titanium compound, is dissolved in the acidic material or the silicate and thus coprecipitated with the silica. A suitable chromium-containing compound for use in this embodiment, for example, is chromic sulfate.
Another method to incorporate a chromium compound into the catalyst, is to use a hydrocarbon solution of a chromium compound convertible to chromium oxide to impregnate the support after it is spray dried or azeotrope dried (i.e., the xerogel). Exemplary of such materials are tert-butyl chromate, chromium acetylacetonate, and the like. Suitable solvents include, but are not limited to, pentane, hexane, benzene, and the like. Surprisingly, an aqueous solution of a chromium compound simply can be physically mixed with the support.
The catalyst system used in the invention must be aged twice, first at a substantially neutral pH and second at an alkaline pH. This twice-aged process is disclosed in U.S. Pat. No. 4,981,831, herein incorporated by reference.
Chromium preferably is present in an amount within a range of about 0.8 to about 3 weight percent, more preferably within a range of about 1.5 to about 2.5 weight percent chromium calculated as CrO3, based on the total weight of the catalyst (support plus chromium compound). These ranges of chromium content provide a catalyst system that is execellent in activity.
Optionally a pore perserving agent can be added during catalyst system preparation, as disclosed in U.S. Pat. No. 4,981,831, herein incorporated by reference.
The resulting twice-aged catalyst system can be dried in any manner known in the art, such as oven drying, spray drying, azeotrope drying, or any other method.
The dried catalyst system then must be calcined. Calcination can take place by heating the dried catalyst system in the presence of an excess of molecular oxygen at a temperature within a range of about 800xc2x0 F. to about 1300xc2x0 F. (about 427xc2x0 C. to about 704xc2x0 C.), preferably about 900xc2x0 F. to 1200xc2x0 F. (about 482xc2x0 C. to about 649xc2x0 C.). Most preferably, the catalyst system calcined at a temperature within a range of about 1100xc2x0 F. to about 1200xc2x0 F. (about 593xc2x0 C. to about 649xc2x0 C.) for about 30 minutes to about 50 hours, more preferably for about 2 to about 10 hours. This calcination procedure results in at least a substanial portion of the chromium in a low valence state to be converted to a hexavalent form. Preferably, this calcination is carried out in a stream of fluidizing air wherein the stream of fluidizing air is contained as the material is cooled.
In order to achieve the desired resultant effects on the resin product, or polymer, the catalyst system must have a low pore volume, usually about 0.5 ml/g to about 1.3 ml/g, preferably about 0.8 ml/g to about 1.2 ml/g. Additionally, the catalyst system must have a low surface area, usually within a range of about 150 m2/g to about 400 m2/g, preferably within a range of about 200 m2/g to 380 m2/g. Most preferably the catalyst system surface area is within the range of 250 m2/g to 350 m2/g.
Catalyst systems of this invention must be used with a cocatalyst. The cocatalyst must be a trialkylboron compound wherein each alkyl group has from about 1 to about 10 carbon atoms, preferably about 2 to about 4 carbon atoms per group. Trialkylboron compounds must be used as a cocatalyst because the compounds are effective agents to improve polymer properties, such as, for example to decrease die swell and to decrease weight swell. By far, the most preferred cocatlyst is triethylboron.
The cocatalyst is used in an amount within a range of about 1 to about 6 parts per million (ppm), or milligram per kilogram (mg/kg), based on the amount of diluent in the reactor. Preferably the cocatalyst is used in an amount within a range of about 2 to about 4 ppm, for cost effectiveness, best polymer properties, and decreasing the amount of smoke resulting from the resin during processing.
Catalyst systems of this invention can be used to polymerize at least one mono-1-olefin containing about 2 to about 8 carbon atoms per molecule, preferably ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, and 1-octene. The invention is of particular applicability in producing ethylene homopolymers and copolymers from mixtures of ethylene and about 0.5 to about 20 mole percent of one or more comonomers selected from the group consisting of alpha-olefins containing about 3 to about 8 carbon atoms per molecule. Exemplary comonomers include aliphatic 1-olefins, such as propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene and other olefins and conjugated or non-conjugated diolefins such as 1,3-butadiene, 1,4-pentadiene, 1,5-hexadiene, and other such diolefins and mixtures thereof. Ethylene copolymers preferably constitute at least about 90, preferably 97 to 99.8 mole percent polymerized ethylene units. With ethylene/1-hexene copolymers, about 98 to 99.8 mole percent ethylene is preferred, the remainder of course being comonomer. Propylene, 1-butene, 1-pentene, 1-hexene and 1-octene are especially preferred comonomers for use with ethylene.
Catalyst systems of this invention must used in slurry polymerization processes. A slurry, or particle form, process generally is carried out in an inert diluent (medium). The diluent useful in the practice of this invention must be isobutane. While other diluents are known, or even can be used, other diluents will not result in the decreased die swell and decreased weight swell as disclosed in this invention.
The temperature of the slurry reactor must be within a range of 200xc2x0 F. to 225xc2x0 F. (93xc2x0 C. to 107xc2x0 C.). Temperatures outside of that range will not result in a polymer having the required resultant properties. Pressures in the particle form process can vary from about 110 to about 700 psi (0.76 to 4.8 MPa) or higher.
The catalyst system is kept in suspension and is contacted with the monomer(s) at sufficient pressure to maintain the isobutane and at least a portion of the monomer(s) in a liquid phase. The isobutane and temperature thus are selected such that the polymer is produced as solid particles and is recovered in that form. Catalyst system concentrations can be such that the catalyst content ranges from about 0.001 to about 1 weight percent, based on the weight of the reactor contents.
Hydrogen can be added to the slurry polymerization to control molecular weight, as is known in the prior art. When used, hydrogen generally is used at concentrations up to about 2 mole percent of the reaction mixture, preferably within a range of about 0.1 to about 1 mole percent of reaction mixture.
Polymers produced in accordance with this invention must be a copolymer of ethylene and at least one higher alpha-olefin. The comonomer, or higher alpha-olefin, is present in the polymerization reactor in an amount within a range of about 0 to about 1.0 mole percent.
Copolymers produced according to this invention have a reduced die swell and a reduced weight swell as compared to conventionally prepared polyethylene copolymer resins. The polymer, or resin product, generally has a density within a range of about 0.95 to about 0.96 g/cc, preferably within a range of about 0.952 to about 0.958 g/cc. Most preferably polymer product density is within a range of 0.954 to 0.956 g/cc. The HLMI of the resultant polymer generally is within a range of about 10 to about 80 g/10 minutes, preferably about 13 to about 40 g/10 minutes. Most preferably, the HLMI is within a range of 15 to 30 g/10 minutes. The sheer response, or HLMI/MI ratio, is within a range of about 100 to about 250, preferably within a range of about 110 to about 200. Most preferably, the HLMI/MI ratio is within a range of 125 to 175.
Polymers produced in accordance with this invention also have a broad molecular weight distribution, as evidenced by the ratio of Mw/Mn. Usually, Mw/Mn, wherein Mw is the weight average molecular weight and Mn is the number average molecular weight, is within a range of about 10 to about 30, preferably within a range of about 12 to about 25. Most preferably, the Mw/Mn is within a range of 15 to 22.
The ESCR of products produced from this resin is greater than about 200 hours, preferably greater than about 500 hours based on testing under Condition A. Most preferably, the ESCR is within a range of about 1000 hours to about 10,000 hours. Further, the resin exhibits low weight swell, which is lower than typical standard blow molding resin such as Phillips Marlex(copyright) polyethylene HHM 5502 or Phillips Marlex(copyright) polyethylene HHM 5202 under Uniloy blow molding conditions. Further, resins produced in accordance with this invention, have a low die swell, which is at least lower than typical standard blow molding resin such as Phillips Marlex(copyright) HM 5502 or Phillips Marlex(copyright) HHM 5202 under Uniloy blow molding conditions.
The normalized die swell of polymers produced in accordance with this invention usually is less than about 0.95, preferably less than 0.90. Most preferably, the normalized die swell of polymers produced in accordance with this invention is less than 0.85 for best polymer process throughput.
The onset of melt fracture for polymers produced in accordance with this invention is greater than about 2000 secxe2x88x921, preferably greater than 2200 secxe2x88x921. Most preferably, the onset of melt fracture for polymers produced in accordance with this invention is greater than 2300 secxe2x88x921 for best polymer processing throughput.
Another way to distinguish polymer products produced from this resin is to compare them to currently commercially available ethylene polymers. For example, relative to a Phillips Petroleum MARLEX(copyright) 5502 polyethylene resin, polymers of the present invention generally have a HLMI less than 90% of the typical values for MARLEX(copyright) 5502, a HLMI/MI ratio of greater than 110% of the standard values for MARLEX(copyright) 5502, a Mw/Mn of greater than about 110% of typical values for MARLEX(copyright) 5502. Additionally, die swell and weight swell of the resins produced in accordance with the present invention are lower than typical values for 5502. The normalized die swell of the inventive resin is generally less than 95% of the normalized typical die swell values for MARLEX(copyright) 5502 and the weight swell of the inventive resin is less than about 90% of standard or typical values for MARLEX(copyright) 5502. However, density of the inventive resin is within the standard ranges of 5502. In addition, the ESCR of the inventive resin is more than two times typical ESCR values for 5502.
The following examples are provided to further assist a person skilled in the art with understanding the invention. The particular reactants, conditions, and other variables are intended to be generally illustrative of these inventions and are not meant to be construed to be unduly limiting the reasonable scope of the invention.